I'm slowly working on installing our dishwasher in a house that didn't have one before. We had an electrician do the wiring and I replaced the sink drains to add a dishwasher wye last week. I am confident on almost all of the rest, but I am a bit stuck on exactly how I should modify my sink's water supply fitting to branch off to the dishwasher.

This is what the existing faucet's hot water supply line looks like, including literal measurements I took of the copper and steel hose outer diameters:

enter image description here

What type and specs of tee/valve do I need to add? And is there any way to do this without cutting into the copper?

I picked up a dishwasher supply hose from the hardware store that has a '3/8" compression to valve inlet' on the end. One thing I'm confused about is that the steel hose is much thicker than the faucet hot water supply hose (measures 1/2") so I'm wondering if it's too thick, or if the supply lines to the faucet are unusually skinny or something. I assume they should have the same diameter?

Thanks in advance for any insight! Have spent way too long thinking about this already!

  • Pipe nominal diameters are INTERNAL (and nominal.) That copper pipe is ... 1/2" copper tubing size. You can look up ID/OD for pipes easily. Hoses are a bit trickier, but if the nut size on the fitting matches (for the same type of fitting) you are generally good to go, there.
    – Ecnerwal
    Aug 9, 2022 at 22:53
  • A hose of ID "x" will always have a larger OD than a pipe of the same ID. The rubber wall of a hose has to be thicker than the copper/steel/PEX/etc wall of other piping because rubber is flexible and it takes more rubber to resist the water pressure inside it than it does copper (or other material).
    – FreeMan
    Aug 10, 2022 at 16:18

2 Answers 2


Part of why you may have spent way too long thinking about it already is that there are literally countless valid ways of solving the problem. But before we get into exploring some of those ways, let's identify your starting point.

The supply tube with measured OD 5/8" diameter is what's called "1/2 inch copper pipe" -- the "nominal size" we use to refer to the pipe frequently does not match the actual measured size. Anyway.. The shutoff valve is "sweated," or soldered, to the supply tube. The size of the outlet fitting is even further disconnected from the physical size of the braided sheath on the faucet's hot water supply line. That outlet is definitely a compression type fitting, but I'm struggling to convince myself whether it's 3/8 or 1/2 inch size. I'm hoping for 3/8" because..

There are some nice two-in-one shutoff valves available that fit your use case very nicely. They use a compression fitting to attach to the incoming 1/2" copper tube and have two 3/8" compression outputs, each controlled by their own valve. There are a few styles available and from multiple manufacturers. One example is shown below. (photo: homedepot.com)

dual-outlet shutoff valve

To install it you'll need to cut off the existing shutoff valve. Though it's a bit more work, I recommend it because shutoff valves tend to become inoperable with age and yours does appear to have some age under its belt!


You should put a "T" in the line with a valve for the dishwasher. From your measurements I would say it is 1/2" trade size. You will have to turn off the water, drain the hot water line you show. Then cut it and solder in a "T" or they also make compression "T's" that you can use if you cannot solder. They are approved in my area but I do not know about yours. In the branch of the "T" you need to place a piece of copper tubing then a valve for the dishwasher. When you purchase that valve take along your hose and get a compatible valve. When you turn on the water gently turn on the valves in the sink, they will sputter as will the rest of the sinks etc in your home until the air is pushed out. Note: When draining you need to open a valve above the drain point which should be below your working area. In our area if we have a garbage grinder the dishwasher drains into that which also serves as a vacuum breaker which you will need.

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